Conventionally, in order to attain precise relative alignment between a reticle and a wafer, the distance (base line amount) between a projection lens and an off-axis observation optical system needs to be precisely calculated.
FIG. 2 is a view schematically showing a method of measuring the base line amount in a projection exposure apparatus. Reference numeral 1 denotes a light source; reference numeral 2, a reticle; and reference numeral 3, a position detection mark arranged on the reticle 2. An illumination optical system 4 uniformly illuminates the reticle 2 with light from the light source 1. A projection lens (projection optical system) 5 projects the pattern of a reticle onto a wafer. Reference numeral 6 denotes a wafer; and reference numeral 7, a wafer chuck to hold the wafer 6. A wafer Z stage 8 can vertically drive the wafer chuck 7. A wafer X-Y stage 9 can hold and drive the wafer Z stage 8 in the X and Y directions, which are parallel to the wafer plane. A wafer stage includes the wafer chuck 7, wafer Z stage 8, and wafer X-Y stage 9. A stage base 10 supports the X-Y stage 9.
A wafer stage position reference mark 11 is positioned to be almost flush with the surface of the wafer 6. A TTL observation optical system 12 detects an image of the wafer stage position reference mark 11, which has returned upon passing through the projection lens 5. Reference numeral 13 denotes a TTL observation optical system arithmetic processor. Reference numeral 14 denotes an off axis observation optical system provided separately from the projection lens 5. Reference numeral 15 denotes an off-axis observation optical system arithmetic processor; and reference numeral 16, a controller to control the overall projection exposure apparatus.
An image of the position detection mark 3 on the reticle 2 is projected onto the wafer stage position reference mark 11 through the projection lens 5. The light reflected by the wafer stage position reference mark 11 is detected by the TTL position observation optical system 12. This makes it possible to calculate the distance from the origin of the wafer stage 8 to the image of the reticle position detection mark 3.
The wafer Z stage 8 is activated to move the wafer stage position reference mark 11 under the off-axis observation optical system 14. The off-axis position observation optical system 14 detects a position detection mark image of the wafer stage position reference mark 11. This makes it possible to calculate the distance from the origin of the wafer stage 8 to the off-axis observation optical system 14.
On the basis of position information of a wafer stage position reference mark 11 measured by the projection lens 5 and that measured by the off-axis observation optical system 14, the distance (base line amount) between the projection lens and the off-axis observation optical system can be calculated.
For alignment precision improvement, it is very important to appropriately control the base line amount in such an exposure apparatus comprising the off-axis observation optical system 14 and TTL observation optical system 12 to control the base line amount, thereby indirectly executing alignment between the reticle and the wafer.
In a best focus position detection method of detecting the position of the image plane of the projection lens 5 using the TTL observation optical system 12, a reference mark on the wafer stage is observed through the projection lens 5, a mark (reticle-side mark) on the reticle or reticle stage and the TTL observation optical system 12. At this time, a focus adjustment process (also to be referred to as reticle-side focus measurement hereinafter) for the TTL observation optical system 12 on the reticle-side mark, and a process (also to be referred to as wafer-side focus measurement hereinafter) of moving the wafer stage in the direction of the optical axis of the projection lens 5 to match the image plane of the reticle-side mark with that of the wafer stage reference mark are executed. To improve the best focus position detection precision, it is very important to appropriately control the precisions of focus measurements on the reticle and wafer sides.
When an exposure process is continuously performed, heat is generated upon driving the reticle and wafer stages and heat due to exposure light irradiation is also generated. A temperature change due to such heat varies the base line and best focus, resulting in deterioration of alignment precision and focus precision. To solve this problem, a method of executing measurement for every wafer exchange or predetermined wafer count is adopted.
As shown in FIG. 15, this measurement method allows base line measurement by causing the off-axis observation optical system 14 and TTL observation optical system 12 to perform measurements at the same base line measurement timing. A measurement timing setting method shown in FIG. 15 will be explained below.
In step 1501, an exposure process is started. The controller 16 determines in step 1502 whether the base line measurement timing is detected. If the base line measurement timing is detected (“YES” in step 1502), the flow advances to step 1503. If no base line measurement timing is detected (“NO” in step 1502), the flow advances to step 1506. In step 1503, the TTL observation optical system 12 performs measurement. In step 1504, the off-axis observation optical system 14 performs measurement. In step 1505, the controller 16 combines the values measured by the observation optical systems to calculate the correction value of a correction item set in a job. The controller 16 controls operation to execute wafer alignment in step 1506 and exposure in step 1507. The controller 16 determines in step 1508 whether exposure for all the shots on the wafer is complete. If exposure for all the shots is complete (“YES” in step 1508), the wafer exposure process is ended. If an unexposed shot remains (“NO” in step 1508), the flow returns to step 1502.
As described above, the conventional measurement method allows the TTL observation optical system 12 and off-axis observation optical system 14 to perform measurements at the same base line measurement timing.
The conventional measurement method executes measurement at a preset design timing regardless of the state of the apparatus.
For example, base line measurement is allowed by causing both the off-axis observation optical system and TTL observation optical system to perform measurements. However, parameters do not necessarily vary so greatly that corrections in both the optical systems are required. A variation on one side may be large and that on the other side may be small.
Similarly, focus measurement systems also suffer from focus variations on the reticle and wafer sides. A variation on one side may be large and that on the other side may be small.
As a result, no measurement timing suitable for each observation optical system is set, so a further improvement in throughput is demanded.